A new study published in Hydrological Processes challenges the common assumption that Himalayan glaciers are the primary source of the Ganga river flow, concluding instead that groundwater aquifers are the main source of the river summer flow as it crosses the plains.
The research led by Dr. Abhayanand Singh Maurya of the Indian Institute of Technology, Roorkee is the first comprehensive isotope study to show this relationship. While glacier and snowmelt contribute significantly in the upstream, mountainous regions, their importance diminishes beyond the Himalayan foothills.
Groundwater as the Primary Source in the Plains
The study found that in the densely populated Indo-Gangetic Plains, groundwater discharge increases the river volume by about 120% compared to its volume at its origin.
• The researchers collected water samples along the river course, from its confluence at Devprayag to near its mouth, and analyzed their oxygen and hydrogen isotopic ratios.
• The change in these isotopic ratios along the river’s path indicates mixing with groundwater, which typically has different isotopic values than surface river water.
• The findings show that for a crucial 1,200 kilometer middle plain segment a region that supports over 400 million people and is vital for agriculture and industry the Ganga is “mostly groundwater-fed.”
Concerns Over Extraction and Evaporation
Although the study confirms that groundwater is the river’s lifeline, the Ganga basin faces severe threats:
• Evaporation Loss: The study revealed that the Ganga loses more than half of its water due to evaporation in its middle plain region.
• Drought and Drying: Recent research has found that the Ganga basin has faced its worst droughts in the last few decades, with “drying from 1991 to 2020 is unmatched in the past millennium.”
• Over-extraction: Researchers identified over-extraction of groundwater for agriculture and urban use, glacial retreat, and excessive water diversion at multiple stages as main disruptors to the river’s flow.
The Need for Groundwater Conservation
Experts affirm that while glaciers are not the largest source of water, they are crucial because they ensure the river remains perennial, especially during lean winter periods.
The findings underscore the need for a renewed focus on conservation, with the study suggesting that river rejuvenation and preservation programs must prioritize groundwater recharge.
Ruchi Badola, Dean at Wildlife Trust India, noted that their conservation plans, including the work of local volunteers (Ganga Praharis), already emphasize conserving the catchment areas to recharge groundwater. She also stressed the need to focus on tributaries which collect water from the fields and discharge it into the Ganga as their health is essential for the sustainability of the main river.
To ensure a sustainable flow the study suggests increasing water release from dams to 20% -25% of the natural flow at key locations, halting untreated sewage discharge, and curbing the dumping of religious materials.
Following the publication of the isotope study, the National Green Tribunal (NGT) initiated suo motu proceedings and has directed the Union Ministry of Environment, Forest and Climate Change to respond to the findings.